Fiberboard is normally manufactured in the form of MDF (Medium Density Fiberboard). This material is a board product which is based on wood, and which in recent years has been used to a rapidly increasing extent. MDF is to be understood here as comprising fiberboard with a varying density, and which is manufactured according to the dry method. MDF is made from wood fibers, which are dried, glued, formed and pressed in a hot press in a known manner. Almost without exception the thus-formed fiber web or mat is a so-called single-layer board, i.e. it has a substantially homogeneous structure with uniform fiber distribution, uniform moisture content, and uniform glue addition across its thickness. The relatively dry surface layer of fluff fibers is thus exposed to both radiant heat and contact heat in the hot inlet to the hot press, which is normally a continuous press.
Particle board is manufactured in a similar way, and is now primarily built up as so-called triple-layer board, i.e. it comprises a central layer of coarse chips and two surface layers of fine chips. These layers are manufactured separately and, therefor, it is also possible to select different moisture contents and glue contents in the various layers. During the manufacture of particle board the surface layer is also exposed to the press heat, but is not dried out with the same intensity, because of the higher moisture content in the surface layers, and because of the more compact nature of the chip material.
During the manufacture of board of these and similar types, as mentioned, a web or mat is formed which is pre-pressed and possibly pre-heated prior to its being fed into the hot press, where the pressing is carried out at a controlled surface pressure and/or thickness at a temperature of between about 150.degree. and 230.degree. C. and where conventional urea formaldehyde glues are used. (Other glues are also used, especially at higher board densities). In order to bring about the necessary board properties, a continuous press is required which is flexible, and at which, among other things, a high surface pressure can be applied at an early stage in the press. This implies at the same time, that the thickness of the mat, even at this early stage, is very close to the final pressed thickness, i.e. the mat thickness must be reduced very substantially in the press inlet.
In order to ensure that such a reduction in thickness takes place without destroying or attenuating the surface layers of the mat, the inlet portion must be long and preferably wedge-shaped, so as to provide the time required for air enclosed in the mat to be transported out of the mat in a gentle way. Such an inlet, however, causes the surface layer to be substantially heated and dried out at a location where the surface pressures required for compressing the mat are still very low. With this method, therefore, the surface particles become dried out, and therefore the glue also dries out and is inactivated, which results in an unsatisfactory hardness and strength of the surface layer. As a result, the surface layer obtained is often called a pre-hardening layer, because the glue therein has hardened and/or been dried out before sufficient surface pressures have arisen to bring about good contact between the fibers or particles. This surface layer must then be ground in a later production step, and therefore constitutes a substantial loss of raw material and a substantial increase in handling.
It should also be mentioned in this connection that the press temperature at the beginning of the press cycle, i.e. in the inlet portion (compression portion) of the press should be as high as possible in order to soften the surface layer as rapidly as possible when surface pressure has been applied, and in order to obtain the highest possible heat penetration rate into the board. This desire is therefore in direct conflict with the complex problem of pre-hardening.
Another factor which promotes the increase of pre-hardening is the fact that the known continuous presses are provided with endless conveying belts made of steel, and these belts require a large radius of curvature, generally on the magnitude of about 800 to 1000 mm, which renders heating in the inlet unavoidable.
On the other hand, when pre-hardening is to be minimized, the aim must be to compress the mat as quickly as possible to a sufficient surface pressure for good bonding. With known designs of continuous presses it is possible to reduce pre-hardening to some extent, but there is instead the risk of attenuations and surface cracks arising in the surface material, because enclosed air, which must be pressed out rapidly, causes an over-pressure to arise in the mat. Such faults may only be discovered, however, at a much later date. Such surface cracks, for example, often can pass unnoticed through the process of board production, and it is only when the board is to be painted by the customer that the surface cracks are found to have caused variations in the surface density, resulting in varying paint absorption or suction, and thus a varying glaze. Without exception, this gives rise to complaints.